Some types of vehicles installed with an engine automatic stop-and-start system, such as an idle reduction control system, have been recently developed for reduction in fuel cost, in exhaust emission, and the like. Such engine automatic stop-and-start systems are designed to control an internal combustion engine, referred to simply as engine, installed in a vehicle so that the engine speed is reduced toward zero in response to a driver's engine stop request. After the stop of the engine, these engine automatic stop-and-start systems are designed to cause, in response to a driver's operation to restart the vehicle, a starter to crank the engine, thus restarting the engine.
In normal starters, a pinion is shifted by an actuator toward a ring gear coupled to a crankshaft of an engine to be engaged with the ring gear. While being meshed with the ring gear, the pinion is rotatably driven by a motor so that the crankshaft is rotated. This cranks the engine.
An example of engine start systems equipped with such a starter is disclosed in Kohyo (National Publication of Translated Version) No. 2008-51009. An engine start (restart) system disclosed in the Kohyo is provided with an electronic first switching element, such as a transistor, for turning on or off the supply of power to the starter motor. The engine start system is also provided with a second switching element for turning on or off the supply of power to the actuator.
Immediately before the engine speed reaches zero or after the engine speed has reached zero, the second switching element is controlled to be turned on. This causes the actuator of the starter to shift the pinion toward a ring gear of an engine so that the pinion is meshed with the ring gear.
After the engagement of the pinion with the ring gear, the first switching element is controlled to be turned on during a preset period. This causes the starter motor to rotate its pinion so that a rotational position (crank angle) of the crankshaft of the engine being off relative to a reference position is set to a target position (target crank angle) suitable for engine-restart. The crank angle of the engine being off relative to the reference position will be referred to as “engine-stop crank angle”.
Thereafter, when an engine restart request occurs, the first switching element is controlled to be turned on so that the pinion being meshed with the ring gear is rotated by the starter motor. This cranks the crankshaft of the engine.
The techniques disclosed in the Kohyo are designed to turn the electronic first switching element on to thereby crank the engine in response to the occurrence of an engine restart request. Relatively high current, for example, within the range of 500 to 1500 amperes, is normally required for a starter motor to crank an engine. For this reason, a higher capacity (current capacity) power transistor through which such a relatively high current can flow needs to be provided as the electronic first switching element. However, because such a higher capacity power transistor is expensive, engine restart systems based on the techniques disclosed in the kohyo cannot meet the low-cost requirements for vehicles as recent important technical requirements.
In addition, electronic switching elements, such as power transistors, are normally operative to control the amount of a current to be supplied to the starter motor with high accuracy. However, mechanical relays, such as electromagnetic relays, cannot control the amount of a current to be supplied to the starter motor because the operation speed of electronic switching elements is faster than that of mechanical relays.
For this reason, if an inexpensive mechanical relay suitable for high-current supplying were provided in place of the first electronic switching element in the engine restart system disclosed in the kohyo, it would be difficult for the mechanical relay to control the amount of a current to be supplied to the starter motor with high accuracy. This would make difficult to set, with high accuracy, the engine-stop crank angle to the target crank angle suitable for engine-restart; this may deteriorate the restartability of the engine, such as reduce a time required to restart the engine.